As the drive power sources for portable electronic equipment such as mobile telephones (including smartphones), portable computers, PDAs, and portable music players, much use is made of alkaline secondary batteries and nonaqueous electrolyte secondary batteries, typified by nickel-hydrogen batteries and lithium ion batteries, respectively. Furthermore, alkaline secondary batteries and nonaqueous electrolyte secondary batteries are also much used as drive power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs), and in stationary storage battery systems in applications for curbing output variation of photovoltaic power generation and wind power generation, etc., in grid power peak load shifting applications for storing power at night and using it in the daytime, and in other applications. Particularly in EV, HEV and PHEV applications or stationary storage battery systems, high capacity and high output characteristics are required. Individual batteries accordingly get larger and are used connected in series or in parallel. Prismatic secondary batteries are widely used in such cases, because of their space efficiency.
In the prismatic secondary batteries used in these applications, it is necessary not only to increase battery capacity but also to achieve high-power output. Large current flows in the battery during discharge at high-power output, and therefore, a reduction in battery internal resistance is required. Thus, a variety of improvements have been made for realizing higher reliability and lower resistance at a joint portion of a terminal portion or inside the battery for the purpose of minimizing the battery internal resistance and eliminating variations in internal resistance.
Mechanical crimping has been often used in the related art for realizing lower resistance at a joint portion of a battery terminal portion or inside a battery. However, with only mechanical crimping, electrical resistance varies over time under environments with frequent vibrations as in EVs, HEVs, PHEVs, and the like. Therefore, the boundary portion of the crimped joint portion is welded with high energy beams such as laser beams, as disclosed in JP-A-2009-087693, JP-A-2008-251411, and JP-A-2010-033766. In this case, only part of the boundary portion is welded in the form of a spot with high energy beams because, the portion under the force of crimping is melted if the boundary portion is entirely welded, and the force of crimping becomes weak. JP-A-2008-251411 and JP-A-2010-033766 show examples in which welding with high energy beams is performed for each of a plurality of regions along the boundary portion of the crimped joint portion such that a plurality of weld spots overlap each other.
Among those examples, a method of forming a joint portion between a collector and a terminal as disclosed in JP-A-2008-251411 will be des bed with reference to FIG. 10, in the case of using laser beams as high energy beams. FIG. 10A is a sectional view showing a step of processing a tip of a crimped portion of a terminal as disclosed in JP-A-2008-251411, FIG. 10B shows a step of laser-welding after the step in FIG. 10A, FIG. 10C is a plan view of FIG. 10B, and FIG. 10D is a plan view after laser welding is repeated a plurality of times such that a plurality of weld spots overlap each other.
A joint portion 60 between a collector and a terminal as disclosed in JP-A-2008-251411 includes a cover plate 61 fixed to a battery outer casing (not shown), an inner insulating sealing material 62 and an outer insulating sealing material 63, a collector 64 connected to a power generating element, and a rivet terminal 65. The inner insulating sealing material 62 and the outer insulating sealing material 63 have a through hole and are disposed at the inner and outer peripheral portions of an opening formed in the cover plate 61. The collector 64 is arranged to overlie the inner insulating sealing material 62. The rivet terminal 65 has a crimped portion 65b projecting from a jaw portion 65a. 
The joint portion 60 is assembled such that the crimped portion 65b of the rivet terminal 65 passes from the outer peripheral side of the cover plate 61 through the outer insulating sealing material 63, the opening of the cover plate 61, the inner insulating sealing material 62, and a rivet terminal hole of the collector 64. Subsequently, the joint portion 60 is integrated by crimping the crimped portion 65b of the rivet terminal 65 so as to press the collector 64. A processing punch A is prepared, which has a concave portion complementary to the crimped portion 65b of the rivet terminal 65 and has a slanted portion A1 at a particular angle at the edge of the concave portion. The processing punch A is then pushed such that the slanted portion A1 abuts on a tip 65c of the crimped portion 65b to partially deform the tip 65c of the crimped portion 65b. As shown in FIG. 10B, the tip 65c of the crimped portion 65b is thus formed into a truncated-cone shape. Consequently, the shape of the tip 65c of the crimped portion 65b is adjusted to form an obtuse angle.
As shown in FIG. 10B and FIG. 10C, laser welding is performed by applying laser beams LB in the vertical direction or the direction therearound on the upper surface of the truncated-cone portion of the tip 65c of the crimped portion 65b. Here, the range of applying laser beams LB is set to include at least the collector 64 and the truncated-cone portion of the tip 65c of the crimped portion 65b, thereby the collector 64 and the truncated-cone portion of the tip 65c of the crimped portion 65b are butt-welded. This laser spot welding enables the uniform transmission of the energy of laser beams applied to both the collector 64 and the truncated-cone portion of the tip 65c of the crimped portion 65b, so that good weld spots (nuggets) 66 are formed at the weld portion.
Furthermore, as shown in FIG. 10D, the collector 64 and the truncated-cone portion of the tip 65c of the crimped portion 65b are butt-welded so that a plurality of weld spots 66 are formed so as to overlap each other along the collector 64 and the truncated-cone portion of the tip 65c of the crimped portion 65b. 
If the method of forming a joint portion as described in JP-A-2008-251411 and JP-A-2010-033766 is employed as a method of forming a joint portion at a battery terminal portion or inside a battery, the internal resistance is reduced, and in addition, the electrical resistance varies less over time even under environments with frequent variations as in EVs, HEVs, PHEVs, and the like. This brings about advantages of achieving higher reliability and lower internal resistance at the joint portion at the terminal portion or inside the battery.
However, the method of forming a joint portion in this manner provides the same configuration both on the positive electrode side and on the negative electrode side, regardless of the difference in constituent material between the positive electrode side and the negative electrode side. Thus, different problems arise between the positive electrode side and the negative electrode side. For example, in nonaqueous electrolyte secondary batteries such as lithium-ion secondary batteries, an aluminum-based metal (aluminum or aluminum alloy) is generally used as a positive electrode plate substrate, and a copper-based metal (copper or copper alloy) is generally used as a negative electrode plate substrate. Thus, to prevent corrosion due to contact of different metals, in general, the positive electrode collector and the positive electrode external terminal are both formed of an aluminum-based metal, and the negative electrode collector and the negative electrode external terminal are both formed of a copper-based metal.
The aluminum-based metal has weak material strength making it difficult to ensure the joining strength only with crimping. Therefore, on the positive electrode side, it is preferable to combine crimp fixing and welding with high energy beams to ensure the joining strength and the electrical continuity. The copper-based metal has strong material strength making it possible to ensure robust joining strength only with crimping. However, it is more preferable to additionally perform welding with high energy beams as in the positive electrode side.
However, as in the examples of the related art, if the physical configuration including the crimp-fixed portion and the weld portion with high energy beams is identical on the positive electrode side and the negative electrode side, that is, the size of each part is identical, the strength of the crimp-fixed portion may be weaker on the positive electrode side than on the negative electrode side. In addition, the strength of the weld portion may be weaker on the negative electrode side than on the positive electrode side because the welding depth is large on the positive electrode side whereas the welding depth is small on the negative electrode side.